1. Field of Invention
The invention relates to a bone anchoring device including a bone anchoring element having a head and a shaft for anchoring in a bone, a receiving part for coupling the bone anchoring element to a rod, and a pressure member having a first surface for engaging the head and a second surface on which the rod acts. The receiving part includes an accommodation space for accommodating the head and a bore being in communication with the accommodation space, the bore having a bore axis, The pressure member is configured to assume a first position in which it exerts a preload onto the head that results from friction between the first surface and the head to enable the shaft to be maintained in a desired angular position before locking the head in the receiving part, and a second position in which the head is locked with respect to the receiving part. The first position may be achieved by moving the pressure member with a predefined force acting onto the pressure member in an axial direction, and the pressure member may be maintained or held at the first position by interaction with the receiving part, and can be released from the first position through action of another axial force.
2. Description of Related Art
US 2007/0118123 A1 describes a polyaxial bone anchor with increased angulation. The polyaxial bone anchor has a locking element shaped and configured to allow an anchoring member, e.g. a screw or hook, to polyaxially rotate at large angles about a central axis of the bone anchor before compression locking the anchoring member within an anchor head.
U.S. Pat. No. 7,604,656 B2 describes an apparatus including a fastener, a housing having a passage, and a spacer received in the passage and engageable with the fastener, wherein pin members retain the spacer and the fastener in the housing and wherein an end portion of the pin members has a tapered surface by which the spacer is urged axially toward the fastener when the pin member is inserted through the housing. The pin members also apply an axial force to the spacer to prevent relative movement between the spacer and the housing when the rod is disengaged from the spacer and the spacer engages the fastener. The pin members hold the spacer in frictional engagement with the fastener.